At the present time, there are a variety of furnaces and heating systems available for heating buildings, particularly houses or single family dwellings.
Commonly, in most developed countries, fossil fuel is used, e.g. oil, gas, coal or coke. The latter two fuels have become less common for domestic use, due to handling problems. It is difficult and expensive to provide any sort of automatic handling and feeding arrangement for such solid fuels. Oil and gas, on the other hand, have the advantage that, being fluids, it is a relatively simple matter to regulate their flow to a furnace or water heater. As a consequence, boilers and water heaters and other heating appliances fueled by oil and gas can be run automatically and left unattended for long periods. Oil and gas can both be stored in relatively large quantities and a combustion chamber configuration for automatic operation can readily be designed.
At the present time, the use of wood as a fuel is regaining popularity in various parts of North America and elsewhere. In many developing countries, and underdeveloped countries, wood is a significant energy source. In appropriate areas, it has the advantage that it is readily available and relatively inexpensive. Typically, in a house or single unit dwelling, the wood is burnt in a fireplace, which may be open or closed, or in a freestanding wood stove. Water heating units can be incorporated, as desired. Wood, however, like coal and coke, suffers from the disadvantage of being a solid fuel. This makes it necessary to manually fuel a heating unit at regular intervals. Also, wood has a relatively low heat value per unit volume, necessitating relatively frequent fueling of the stove or furnace.
The requirements of a particular heating situation will determine the operating power output desired of the solid fuel appliance. For convenience, the wood-burning appliance must require only infrequent refuelling. In a conventional wood-burning furnace, it is quite difficult to maintain both an efficient fire and combustion for any length of time. Pellet-burning appliances are an exception, as they mostly use an auger to transport fuel from a fuel store to the combustion chamber. Such an arrangement is not suitable for wood in the form of logs.
In the majority of contemporary log-burning devices, the primary combustion chamber also serves as the sole integral fuel store. To provide sufficient fuel store capacity requires an increase in the size of the primary combustion chamber far beyond the optimum size for efficient combustion at the desired operating power output.
Efficient combustion requires temperatures high enough to achieve ignition of the volatile (i.e. non-carbon) fuel constituents. This usually means that the fuel everywhere in the combustion chamber will all be burning at approximately the same stage of combustion at all times. When the combustion chamber is sized to provide an adequate fuel store, there is far too much fuel for efficient simultaneous combustion at the desired power output. Thus, operating a large combustion chamber with a restricted air supply and low combustion rate, to give longer periods between refuelling, simply results in inefficient combustion. Inefficient combustion brings increased fuel costs and defeats the desired objective of infrequent refuelling. Also, because uncombusted material is deposited in the flue or exhausted to atmosphere, it brings greater flue fire hazard and increased pollutant emissions.
The problem with log-burning devices is to provide a fuel store that is separate from the combustion chamber, but integral with the appliance as a whole, and to provide a simple and reliable feed to the combustion chamber.
U.S. Pat. No. 528,958 shows rotary elements for grate of a boiler furnace, but is not concerned with the feeding of fuel to the furnace.
U.S. Pat. Nos. 794,853 and 1,152,363 rely on gravity feeding, but do not address the problem of causing fuel to move downwards.
U.S. Pat. No. 183,797 shows a conveying weighing mechanism for charging machines. As such, it includes two conveyor belts whose top surfaces move towards one another.
A variety of feeding mechanisms are disclosed in some other patents. Thus the U.S. Pat. No. 1,935,876 has two hoppers with two feed rollers, which incidentally rotate in the same direction. In U.S. Pat. No. 2,106,469, cores or briquettes are discharged from receptacles onto a conveying mechanism. The conveying mechanism has belts or the like on opposite sides for driving the cores.
U.S. Pat. No. 2,214,740 shows a stoker which, as shown in FIG. 4 of that patent, has a single feed roller.
A somewhat complex feed arrangement is disclosed in the heating device of U.S. Pat. No. 4,126,119. Here, logs are fed axially into the combustion chamber.
A somewhat complex feeding arrangement is disclosed in the Grossniklaus U.S. Pat. No. 4,185,567. This is of some interest in showing the complexity that can arise when a feed other than gravitational is employed. Grossniklaus relies upon the arrangement of a ram and the fuel store beside the combustion chamber to keep the combustion gases out of the fuel store. Three separate flaps are provided for displacing the wood towards a ram, and the ram drives the wood through a feed duct into the combustion chamber.
Another complex feeding mechanism is disclosed in the U.S. Pat. No. 4,444,538. Here again logs are fed axially into the combustion zone, and the log magazine or storage is separate from the combustion zone.
Both Godbout U.S. Pat. No. 4,530,289 and Steindal U.S. Pat. No. 4,606,282 patents are of some interest, in showing a log storage magazine, which is not completely separate from the combustion chamber. In Godbout, chains are used to hold the logs at the bottom of the chamber, and air nozzles supply air to the surfaces of the logs, to generate combustion. The top of the log chamber is closed, to prevent combustion proceeding upwards through the logs. Similarly, in the Steindal patent, a log magazine is closed, so that combustion gases are forced to travel in the desired path, and not up through the logs. Here, two separate chambers are provided in a U-shape. The logs then fall under gravity to the combustion zone.
Accordingly, it is necessary to provide a wood-burning appliance which is designed for infrequent refuelling, yet provides efficient combustion at the desired power output.